The present invention relates to processing a wireless communication signal, and more particularly, to methods and apparatuses for dealing with spectrum inversion of a wireless communication signal complying with a digital terrestrial television multimedia broadcasting (DTMB) standard, such as a time domain synchronous orthogonal frequency-division-multiplexing (TDS-OFDM) signal or a single carrier signal.
Quadrature amplitude modulation (QAM) is a modulation scheme which conveys data/information by modulating the amplitude of two carrier waves (e.g., sinusoid waves) out of phase with each other. In addition, some other modulation schemes can be regarded as special cases of QAM. For instance, phase-shift keying (PSK) can be regarded as a special case, where the magnitude of the modulated signal is a constant, with only the phase varying. In general, when transmitting two signals by modulating them with QAM, the transmitted signal S(t) will be of the following form:S(t)=I(t)cos(w0t)+Q(t)sin(w0t)=I(t)cos(2πf0t)+Q(t)sin(2πf0t),where I(t) and Q(t) are data to be transmitted, and f0 is the carrier frequency.
In the receiver end, a down-conversion is employed to down-convert the received radio-frequency (RF) signal, i.e., S(t), into an intermediate frequency (IF) signal or a baseband signal. Suppose that a mixer is implemented to down-convert the RF signal S(t) into an IF signal using a local oscillation signal LO=cos((w0+v)t). After the mixer output is filtered to remove high-frequency components, the resultant signal will be of the form: I(t)cos(−vt)+Q(t)sin(−vt). In another case where the mixer down-converts the RF signal S(t) into an IF signal using a local oscillation signal LO=cos((w0−v)t), the resultant signal will be of the form: I(t)cos(vt)+Q(t)sin(vt).
A phenomenon, called spectrum inversion, might occur due to the transmitter end failing to transmit the real part and the imaginary part correctly or the architecture of the receiver end. As shown in FIG. 1, the desired signal transmitted from a transmitter end should have the spectrum TX_1. In this example, the center frequency of the transmission signal carrying the desired data is located at 666 MHz. However, if the transmitter end does not transmit the real part and the imaginary part properly, the actual signal transmitted from the transmitter end will have a different spectrum TX_2. As clearly shown in FIG. 1, the direction of the spectrum TX_2 is opposite to that of the spectrum TX_1. Thus, the afore-mentioned spectrum inversion happens. Regarding signal reception of the transmission signal (i.e., an RF signal) generated from the transmitter end, the received signal at a receiver end is converted to an intermediate frequency band. For example, the desired intermediate frequency is 36 MHz. As mentioned above, the local oscillation signal might affect the down-conversion result. Suppose that the spectrum direction is not inverse due to the architecture of the receiver end, the IF signal corresponding to the transmission signal with the spectrum TX_1 therefore has the spectrum IF_1 as shown in FIG. 1, and the IF signal corresponding to the transmission signal with the spectrum TX_2 therefore has the spectrum IF_2 as shown in FIG. 1. Next, the IF signal is processed in the following stage for data extraction. Based on the actual implementation of the receiver end, the IF signal will be down-converted to a baseband through mixers using adequate local oscillation signals. As shown in FIG. 1, the extracted signal corresponding to the IF signal with the spectrum IF_1 has the baseband spectrum Demod_1, whereas the extracted signal corresponding to the IF signal with the spectrum IF_2 has the baseband spectrum Demod_2 whose direction is opposite to that of the spectrum Demod_1. In this example, when the receiver end receives the transmission signal with the desired spectrum TX_1, the real part and imaginary part intended to be transmitted from the transmitter end can be correctly recovered from the extracted signal with the spectrum Demod_1. However, when the same receiver end receives the transmission signal with the inverse spectrum TX_2, the real part and imaginary part intended to be transmitted are not correctly recovered from the extracted signal with the spectrum Demod_2 due to spectrum inversion. In other words, the spectrum inversion would make the receiver end fail to decode the received input correctly, which degrades the performance of the receiver end greatly.
Therefore, method and apparatus capable of detecting the spectrum direction quickly and making the DTMB receiver adapted to the correct spectrum direction efficiently are needed.